Letters on England eBook

But all these wonders are merely but the opening of
his discoveries. He found out the secret to
see the vibrations or fits of light which come and
go incessantly, and which either transmit light or
reflect it, according to the density of the parts
they meet with. He has presumed to calculate
the density of the particles of air necessary between
two glasses, the one flat, the other convex on one
side, set one upon the other, in order to operate
such a transmission or reflection, or to form such
and such a colour.

From all these combinations he discovers the proportion
in which light acts on bodies and bodies act on light.

He saw light so perfectly, that he has determined
to what degree of perfection the art of increasing
it, and of assisting our eyes by telescopes, can be
carried.

Descartes, from a noble confidence that was very excusable,
considering how strongly he was fired at the first
discoveries he made in an art which he almost first
found out; Descartes, I say, hoped to discover in
the stars, by the assistance of telescopes, objects
as small as those we discern upon the earth.

But Sir Isaac has shown that dioptric telescopes cannot
be brought to a greater perfection, because of that
refraction, and of that very refrangibility, which
at the same time that they bring objects nearer to
us, scatter too much the elementary rays. He
has calculated in these glasses the proportion of
the scattering of the red and of the blue rays; and
proceeding so far as to demonstrate things which were
not supposed even to exist, he examines the inequalities
which arise from the shape or figure of the glass,
and that which arises from the refrangibility.
He finds that the object glass of the telescope being
convex on one side and flat on the other, in case
the flat side be turned towards the object, the error
which arises from the construction and position of
the glass is above five thousand times less than the
error which arises from the refrangibility; and, therefore,
that the shape or figure of the glasses is not the
cause why telescopes cannot be carried to a greater
perfection, but arises wholly from the nature of light.

For this reason he invented a telescope, which discovers
objects by reflection, and not by refraction.
Telescopes of this new kind are very hard to make,
and their use is not easy; but, according to the English,
a reflective telescope of but five feet has the same
effect as another of a hundred feet in length.

The labyrinth and abyss of infinity is also a new
course Sir Isaac Newton has gone through, and we are
obliged to him for the clue, by whose assistance we
are enabled to trace its various windings.

Descartes got the start of him also in this astonishing
invention. He advanced with mighty steps in
his geometry, and was arrived at the very borders
of infinity, but went no farther. Dr. Wallis,
about the middle of the last century, was the first
who reduced a fraction by a perpetual division to
an infinite series.